A dispersive element is incorporated with an optical sensor as a spectrometer. The dispersive element includes a guided-mode resonance filter having a plurality of resonance regions. The resonance regions respectively have different filter characteristics, each reflecting a first light beam of a tested light source or transmitting a second light beam of the tested light source light source to the optical sensor, wherein the wavelength of the first light beam is different from that of the second light beam. In one embodiment, the dispersive element is incorporated with an optical sensor to form a miniature, high-resolution and low-cost spectrometer. The spectrometer makes use of the transmission efficiencies of the resonance regions of the guided-mode resonance filter and the light intensity distribution detected by the photosensitive regions of the optical sensor to acquire the spectral data of the tested light source.
